Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/147079
Contamination Control BACK PAGE BASICS WES CASH NORIA CORPORATION Using LABYRINTH Seals to Control Contaminant INGRESSION During my travels to various facilities across the United States, I am often asked about the best way to keep contaminants out of a machine. I usually explain that it begins when the lubricants are received and how they are handled, and continues until they are put into service. At that point it becomes a function of limiting contaminant ingression. Simple modifications to breathers and sight glasses can be beneficial, but shaft seals are just as important in this process. Labyrinth-type seals are common devices for contaminant exclusion, and when properly maintained, they can greatly influence the reliability of the components in which they are installed. A seal's function is multifaceted. Not only is it attached to a machine to keep particles and outside contaminants from getting in, but it also must keep what is inside the machine — whether it is oil, process fluid or a barrier fluid — inside. One study by Exxon Mobil showed that the amount of oil lost is four times the capacity of the machine. The need for minimizing leakage while prolonging the machine's operating life is more critical today, as the demands for reliability and the penalties for failure continue to rise. Standard lip seals will limit certain contaminants from entering and restrict some leakage, but they are subject to failure, especially in systems with high pressure or extreme ambient conditions. One limiting factor of these types of seals is the construction material, with each elastomer having its own strengths and weaknesses. The basic concept of labyrinth seals is to reduce contaminant ingression not only by restricting the clearance through which particles enter but also by creating areas of turbulent flow to SEAL MATERIAL BUNA-N (NITRILE) NEOPRENE Temperature Range -40°F to 225°F -40°F to 225°F Strength Excellent resistance to petroleum Good for refrigerants and ammonia Weakness Low resistance to sunlight and weather Degrades in the presence of some ester-based oils exclude contaminants. These types of systems provide better particle exclusion and less lubricant leakage than standard lip seals, which contact the shaft and rub the surface during operation. Labyrinth seals are constructed in multiple pieces. One piece, known as the stator, is affixed to the machine and remains stationary. The other piece, called the rotor, is attached to the shaft In areas of high airborne particulate matter or heavy washdown areas, a labyrinthstyle seal can help limit the ingress of contaminants and prolong machine life. and rotates with the motion of the system. The two pieces interlock to effectively seal out the majority of contaminants while holding in the lubricant or process fluid. These types of seals are non-contacting, which means that the two opposing faces do not touch each other. They are also considered frictionless, as the rotor and stator are separated by a very small gap. In many instances, this gap can be as small as 0.003 inches, which equates to roughly 76 microns. This may sound like a large opening in comparison to the 1-micron lubricant film inside a rolling-element bearing, but because of the path that the contaminant must take to actually reach the bearing, this TETRAFLUOROFLUOROCARBON gap is more than adequate. ETHYLENE (TFE) (VITON) All contaminants trying to -100°F to 400°F 0°F to 400°F enter the bearing housing must go Compatible with Compatible with through the seal's maze (labyrinth) all common fluids many different of turns and angles in order to synthetic fluids reach the bearing. On the path through all of these turns, particles Low elasticity Breaks down in are constantly subjected to centrifleading to breakthe presence of age during install strong caustics ugal forces due to the rotational motion of the shaft, and only a A comparison of various seal materials (Ref. Seal & Survive, Seal School Handbook) 54 | July - August 2013 | www.machinerylubrication.com